1. Field of the Invention
The present invention relates to an adjustable temperature switch. More particularly, the present invention relates to a concealed adjustable temperature switch for an electric appliance such as a hot melt glue gun for preventing damage.
2. Description of the Related Art
A typical hot melt glue gun comprises a concealed adjustable temperature switch in a body thereof for adjusting the temperature for melting a solid glue stick into liquid. FIG. 5 of the drawings is a conventional hot melt glue gun comprising a body 4′ and a concealed adjustable temperature switch 1′ that is connected by wires 2′ to a heating member 3′. The adjustable temperature switch 1′ includes an adjusting block 11′ with an outer threading 111′, a fixed plate 12′ with a screw hole 121′, a bimetal plate 13′, and a metal plate 14′. The bimetal plate 13′ includes a first plate and a second plate that are bonded together, wherein the bimetal plate 13′ has different bending extents at different temperatures. The outer threading 111′ of the adjusting block 11′ is threadedly engaged with the screw hole 121′ of the fixed plate 12′. The bimetal plate 13′ is electrically connected to an electrode, and the metal plate 14′ is electrically connected to the other electrode.
A tool T can be extended into the body 4′ to turn the adjusting block 11′, moving the adjusting block 11′ toward or away from the bimetal plate 13′ to thereby cause the bimetal plate 13′ to be in tight contact or loose contact with the metal plate 14′. When the adjusting block 11′ is turned in a direction, the adjusting block 11′ moves toward the bimetal plate 13′ and causes the bimetal plate 13′ to be in loose contact with the metal plate 14′. In this case, slight bending of the bimetal plate 13′ at a relatively low temperature is sufficient to cause disengagement of the bimetal plate 13′ from the metal plate 14′, resulting in an open circuit. Thus, electricity supply to the heating member 3′ is cut off.
On the other hand, when the adjusting block 11′ is turned in a reverse direction, the adjusting block 11′ moves away from the bimetal plate 13′ and causes the bimetal plate 13′ to be in tight contact with the metal plate 14′. In this case, greater bending of the bimetal plate 13′ at a relatively high temperature is required to cause disengagement of the bimetal plate 13′ from the metal plate 14′ to cut off electricity supply to the heating member 3′.
However, since the adjustable temperature switch 1′ is concealed in the body 4′, the user could not know the contact between the bimetal plate 13′ and the metal plate 14′. In a case that the user misjudges a tight contact as a loose contact and turns the adjusting block 11′ in the tightening direction, the bimetal plate 13′ will be in an overtight contact with the metal plate 14′ such that great bending of the bimetal plate 13′ still could not cause disengagement from the metal plate 14′. As a result, overheating of the heating member 3′ occurs, leading to overload and damage. Further, excessive rotation of the adjusting block 11′ may cause disengagement from the screw hole 121′ of the fixed plate 12′, leading to falling of the adjusting block 11′. On the other hand, if a loose contact is misjudged as a tight contact, the adjusting block 11′ is turned in the wrong direction such that the bimetal plate 13′ could not be in contact with the metal plate 14′, resulting in an undesired open circuit.
Applicant's U.S. patent application Ser. No. 10/961,196 filed on Oct. 12, 2004 discloses a concealed adjustable temperature switch electrically connected to a heating member. The concealed adjustable temperature switch comprises a fixed plate with a screw hole, an adjusting block having an outer threading threadedly engaged with the screw hole of the fixed plate, a bimetal plate that has different bending extents at different temperatures, and a metal plate. The bimetal plate is electrically connected to one of two electrodes, and the metal plate is electrically connected to the other electrode. A positioning ring is securely mounted on the adjusting block to turn therewith. The positioning ring is fixed on the adjusting block by a fixing ring. The positioning ring includes a projection. Two spaced stops are provided to stop the projection of the positioning ring, thereby limiting turning of the adjusting block. The positioning ring, the fixing ring, and the stops prevent the adjusting block from disengaging from the fixed plate and prevent the electric appliance from being damaged. However, in a case that the hot melt glue gun is disassembled, the positioning ring is disengaged from the stops such that the projection of the positioning ring could not be stopped by either stop. Unintentional rotation leading to overtight contact between the bimetal plate and the metal plate might occur, resulting in overheating and damage.